You cannot
each new LTSC/LTSB release require new licence
the only exception is upgrading LTSC 2019 KMS to LTSC 2021 KMS
P.S. that LTSB "digital license" is not authentic
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The summary of the issue: Windows 10 Enterprise 2021 LTSC doesn't recognize a valid Windows 10 Enterprise 2016 LTSB license key and shows error code 0x80041014.
The long story:
I have bought a used Intel Compute Stick stk2m364cc that doesn't come with Windows from the factory (no OEM), but the previous owner has installed Windows 10 Enterprise 2016 LTSB on it.
Its activation status says it's a "digital license" (not linked to a Microsoft account). Also, I was able to extract the product key.
I would like to upgrade to the latest LTSC, so I have obtained en-us_windows_10_enterprise_ltsc_2021_x64_dvd_d289cf96.iso
According to some articles, I should be able to install the update for free because: "During the life of a LTSC release, you can upgrade your devices to the next or latest LTSC release free of charge using an in-place upgrade."
And LTSC is just renamed LTSB and essentially the same thing.
Here's what I did.
At first, I did a clean install of Windows 10 Enterprise 2021 LTSC, and it did not accept the key. This was my mistake, I should perform an in-place upgrade, as suggested.
So, I decided to step back and do an in-place upgrade. I obtained en_windows_10_enterprise_2016_ltsb_x64_dvd_9059483.iso and reinstalled it clean. It accepted the license key and activated it without any problems, so we can assume the key itself is good.
Then I launched the en-us_windows_10_enterprise_ltsc_2021_x64_dvd_d289cf96.iso installer from within Windows. I opted for a clean install removing personal data and applications. Was it wrong? Should I keep them to ensure the activation upgrade? Anyway, the install process worked almost till 70%, and then it suddenly popped up a small message box with the text: "Windows 10 installation has failed". No error code, nothing.
Looking for a fix, I found some articles explaining there's a Windows Setup problem with integrating updates during installation, and I should install with Internet disconnected. So I disabled my network connection and launched setup again. This time it was completed without any problems. It rebooted, I completed the setup, connected to Wifi, and went to activation. Initially, the status showed it couldn't connect to organization's activation servers. I ignored the error because 2016 LTSB also showed me the same error and it went away after I entered the key. However, 2021 LTSC tries to use the key and fails with a message that the key did not work, error code 0x80041014.
So, how do I update from Windows 10 Enterprise 2016 LTSB to Windows 10 Enterprise 2021 LTSC with the same license key? It should be possible without any local KMS, if 2016 was able to be activated without it, right?
You cannot
each new LTSC/LTSB release require new licence
the only exception is upgrading LTSC 2019 KMS to LTSC 2021 KMS
P.S. that LTSB "digital license" is not authentic
The "disable internet connection" helped me to install windows 10 LTSC 2021 on top of windows 10 LTSB 2016, thank you.